Classification and Taxonomy

Earth holds millions of species, so biologists need an organized system to name them, group them, and study how they relate to one another. Taxonomy is the science of classifying living things into a nested hierarchy of ranks, based mostly on shared characteristics and shared ancestry. A clear classification system lets a scientist in Brazil and a scientist in Japan know they are studying the exact same organism, even if their everyday languages have no word in common.

The domain is the broadest, most general rank in classification. Every organism on Earth belongs to one of three domains, sorted by cell type and basic biochemistry. Bacteria and Archaea are both made of prokaryotic cells, cells with no nucleus and no membrane-bound organelles, but they differ sharply in cell wall chemistry, membrane lipids, and genetics. Eukarya contains every organism built from eukaryotic cells, cells that do have a nucleus and membrane-bound organelles.

Within Eukarya, and within each domain generally, organisms are sorted through a nested series of ranks, from the broadest group down to the most specific. Each rank sits fully inside the one above it. Every organism in a Genus shares that same Family, Order, Class, Phylum, Kingdom, and Domain, but a Genus can hold several different Species.

Binomial nomenclature is the two-part scientific naming system Linnaeus introduced, and every recognized species has exactly one. The name always has two parts: the genus name first, Capitalized, and the species name second, lowercase. In print, scientific names are italicized. Once a name has been written in full once in a piece of writing, later mentions can abbreviate the genus to a single capital letter followed by a period. Example: The gray wolf's scientific name is *Canis lupus*. Genus Canis, species lupus. After the first full mention, it can be shortened to *C. lupus*. Example: The bacterium *Escherichia coli* is almost always abbreviated *E. coli* after its first mention, even in casual writing, because the full name is long and the organism comes up so often.

Taxonomists group organisms by shared characteristics, and today those characteristics fall into two main kinds. Anatomical evidence compares body structures. Homologous structures are body parts that share the same underlying structure and the same evolutionary origin, even if they now do different jobs, such as a human arm, a bat wing, and a whale flipper, which all share the same basic bone arrangement inherited from a common ancestor. Molecular evidence compares DNA and protein sequences directly. Two species with more similar DNA sequences share a more recent common ancestor, and modern taxonomy leans heavily on this kind of comparison to confirm or correct groupings that were first made from anatomy alone.

Here is the full classification of the gray wolf, *Canis lupus*, read from the broadest rank down to the most specific.

Example: Compare the gray wolf (*Canis lupus*) to the domestic dog (*Canis familiaris*). They share the same Domain, Kingdom, Phylum, Class, Order, Family, and Genus, and only split apart at Species, which shows they are extremely close relatives. Now compare the gray wolf to a human (*Homo sapiens*). Both share Domain Eukarya, Kingdom Animalia, Phylum Chordata, and Class Mammalia, but they split apart at Order, wolves in Carnivora and humans in Primates. That earlier split tells you humans and wolves share a more distant common ancestor than a wolf and a dog do.

Using dichotomous-key.txt and specimen-traits.csv: 1. Walk specimen 1 through the key from Step 1. What is its identification? 2. Walk specimen 4 through the key from Step 1. What is its identification? 3. Specimens 6 and 7 both have no wings, live on land, and don't have 8 legs, so both reach Step 6. Which trait and value separates them there, and what is each one's identification? 4. Follow the key until it names a specimen "Rockshell." Which specimen number is it, and what two trait values (in the order the key checks them) led there? 5. Two specimens have wings. Identify both of them, and state the single trait (and its two different values) that tells them apart at Step 2.